ENSENA Informe resumido

Quantum entanglement is a fascinating property of quantum particles that are in a common, highly correlated state, even if they are far separated. While these correlations initially were a much debated scientific curiosity, we now know that they will enable technologies of the future such as the quantum computer and secure secret encryption. Far from being really practical, however, the traditional approaches to creating entanglement are cumbersome and imperfect.

The ERC-funded project “Entanglement from Semiconductor Nanostructures” (EnSeNa) developed novel techniques for creating entangled photon pairs. In particular it pushed the creation of entanglement from semiconductor systems to the next level. In their project, the researchers work towards efficient and controlled sources of entanglement that can be miniaturized and will demonstrate how these sources can be interfaced to other technologies.

The main question was whether semiconductor nanostructures could be designed and utilized in a way so that unwanted effects caused by imperfections could be suppressed well enough to bring forward the quantum signatures. The project tried this for different specific systems, microcavity exciton-polaritons and single semiconductor quantum dots. Both are made from ordinary semiconductor materials that are commonly used to make lasers for laser pointers, DVD players, printers and optical communication equipment.

EnSeNa was very successful in several ways. For one the project established a new technique to excite single quantum dots – nanometer-sized regions of semiconductor material – resulting in extremely pure and even entangled output. This enables high quality single photon sources, which have applications in quantum cryptography and precision optical measurements. For interfacing these single photons to standard nonlinear optics the researchers further engineered a traditional entangled photon pair source to deliver high precision output that is compatible with the quantum dot-derived photons. Using this excitation technique they managed to entangle the output photons in a new way for quantum dots, so called time-bin entanglement. In this case a photon pair created in a superposition of two time slots (time bins) so that no one can tell whether the pair comes in the earlier or later time-bin. This is the most robust coding of quantum information, if we want to send it through an optical fiber, through which most of our regular communication is transmitted today.

Microcavity-polaritons are half-light half-matter quasiparticles by virtue of a microscopic resonator that makes light interact very strongly with matter. Polaritons are thus very efficiently generated and detected using optical techniques and yet they behave like matter in many of their properties. In particular they can scatter like billiard balls and in the process acquire entanglement. In project EnSeNa new structures were designed and studied that could bring this entanglement out of the noise and make it visible and potentially useful, too. Indeed for the first time the research team of EnSeNa managed to show nonclassical correlations between polaritons that emerge from such a scattering process.

Apart from these intended goals several other results were achieved, two of which are highlighted here. From nanowire quantum dots the researchers managed to not only extract entangled photon pairs but even triplets of photons. In another part of their work they created waveguides in which entangled photon pairs are created at the optimum wavelength for transmission through the telecom network. EnSeNa established the research group in Innsbruck and made it a vital part of the new research area of quantum photonics – using the quantum properties of light to lead us to the technologies of the future.